1. Field of the Invention
The present invention relates to a crystal oscillator, and more particularly to a temperature compensated crystal oscillator used as a reference frequency oscillator in a mobile communication device (such as a car telephone, a portable telephone, and a cordless telephone), a satellite communication device, and the like.
2. Description of the Related Art
In recent years, there has been increasing demand for a crystal oscillator used for oscillating a reference frequency in a mobile communication device (such as a car telephone, a portable telephone, and a cordless telephone, and often called a cellular telephone), a satellite communication device, and the like, as such communication devices have been become more and more prevalent.
A crystal oscillator to be used for oscillating a reference frequency is required to have as small fluctuation of the oscillating frequency as possible against changes of temperature. In the case of use in a terminal for an analog portable telephone system used in North America, for example, the fluctuation of the oscillating frequency is required to be within a range of .+-.2.5 p.p.m. over a temperature range of -30.degree. C. to +75.degree. C. For use in a terminal for a narrow-band TDMA type digital portable telephone system, the fluctuation of the oscillating frequency is required to be within a range of .+-.1.5 p.p.m. over a temperature range of -20.degree. C. to +85.degree. C. For use in a terminal for a large capacity analog portable telephone system used in Japan, the fluctuation of the oscillating frequency is required to be within a range of .+-.1.0 .p.p.m. over a temperature range of -20.degree. C. to +85.degree. C. Moreover, such crystal oscillators used in a portable telephone must satisfy other requirements, e.g. miniaturization of the device and reduction of the manufacturing cost, which are particularly eminent requirements for a crystal oscillator used in a cellular telephone. Therefore, a crystal oscillator suitable for such use is strongly needed.
The oscillating frequency-temperature characteristics of a crystal oscillator depend directly on the resonant frequency-temperature characteristics of a quartz crystal resonator used in the crystal oscillator. Therefore, when the oscillating frequency is required to be highly stable over a wide temperature range, a temperature compensation circuit is usually required. There are two known methods for temperature compensation using a temperature compensation circuit. One is an analog temperature compensation method in which change in the impedance of the quartz crystal resonator is compensated by using a temperature sensitive element such as a thermistor. The other is a digital temperature compensation method in which a voltage for compensating the temperature characteristics of the quartz crystal resonator (which is used for the oscillation purpose) in accordance with compensation data prestored in a memory is applied to a variable capacitor, based on information obtained from a separately incorporated temperature detecting quartz crystal resonator.
Hereinafter, conventional temperature compensated crystal oscillators will be described with reference to the accompanying drawings. FIG. 19 is a block diagram showing a conventional analog-type temperature compensated crystal oscillator. As is shown in FIG. 19, a quartz crystal resonator 11 and a temperature compensation circuit 111 are connected in series to an oscillation circuit 12 having an output terminal 13. The temperature compensation circuit 111 includes a circuit portion in which a thermistor 112, a resistor 113, and a capacitor 114 are connected in parallel as well as a circuit portion in which a capacitor 117 is connected in parallel to a thermistor 115 and a resistor 116 that are connected in series. When the ambient temperature changes, the resistance of each of the thermistors 112 and 115 varies, whereby the reactance of the impedance of the whole temperature compensation circuit 111 is varied. Therefore, in the case where the respective values of the thermistors 112, 115, resistors 113, 116, and capacitors 114, 117 are determined so as to compensate the fluctuation of the oscillating frequency based on the changes of impedance, it becomes possible to stabilize against temperature changes the oscillating frequency of the crystal oscillator. Examples of such conventional crystal oscillators are disclosed in Japanese Laid-Open Patent Publication Nos. 55-125702, 56-68002, etc.
FIG. 20 is a block diagram showing a conventional digital-type temperature compensated crystal oscillator. As is shown in FIG. 20, the crystal oscillator includes a digitally controlled crystal oscillator 120, a temperature sensor 121, an A/D (Analog/Digital) convertor 122, and a memory 123. The digitally controlled crystal oscillator 120 has a quartz crystal resonator 11, an oscillation circuit 12, and a variable reactance circuit 124. This crystal oscillator operates as follows: The temperature sensor 121 detects a change in the ambient temperature. The A/D convertor 122 converts the change in the ambient temperature to a digital signal. The compensation data prestored in the memory 123 is read out in accordance with the digital signal. The reactance value of the variable reactance circuit 124 is changed based on the compensation data that has been read out, whereby the oscillating frequency of the digitally controlled crystal oscillator 120 is stabilized against changes in temperature. The compensation data prestored in the memory 123 is in the form of, for example, a table of 8 bits (corresponding to 256 points in temperature) by 7 bits (corresponding to 128 control amounts), as is described in 41st Annual Frequency Control Symposium, 1987, p. 435, `A Digitally Compensated TCXO Using A Single Chip LSI`: by T. Hara, T. Kudo, S. Uriya, H. Saita, S. Ogou, and Y. Katsuta.
However, each of the above-mentioned conventional analog-type temperature compensated crystal oscillator and conventional digital-type temperature compensated crystal oscillator has the following inherent problems.
An analog-type crystal oscillator in which thermistors are used has the following problems. Since a conventional analog compensation method can compensate only some specific fluctuation patterns of the resonant frequency-temperature characteristics of quartz crystal resonators, it is impossible to compensate all the kinds of the fluctuation patterns of the resonant frequency-temperature characteristics of the quartz crystal resonator, thus resulting in poor temperature compensation accuracy. Moreover, the thermistors have a large variation in their characteristics. Therefore, it is required to select quartz crystal resonators having minimum fluctuation of resonant frequency-temperature characteristics. Also, the conventional analog compensation method requires rather complicated calibration. As a result, conventional analog-type crystal oscillators require large production costs.
More specifically, the resonant frequency-temperature characteristics depend largely on the cut angle of the individual quartz crystal resonator. Commercially available quartz crystal resonators have a variation in cut angles of approximately .+-.5 minutes. Such variation in cut angles, however, results in too large a fluctuation in the resonant frequency-temperature characteristics of the crystal vibrators; an analog-type temperature compensated crystal oscillator having such a quartz crystal resonator is not capable of completely compensating changes in temperature. Therefore, it is required to select, from commercially available quartz crystal resonators, those with a cut angle variation of approximately .+-.1 minutes, which results in an increase in the production cost. Moreover, such devices as thermistors are not suitable for being integrated into ICs (Integrated Circuits), which leads to the problem that miniaturization of the crystal oscillator can be difficult.
On the other hand, a digital-type temperature compensated crystal oscillator corrects the frequency with respect to each sampling point in temperature, which requires compensation data consisting of a table of 8 bits by 7 bits, for example. The compensation data must be stored in a PROM (Programmable Read Only Memory). Therefore, digital-type temperature compensated crystal oscillators require a great amount of time and trouble in the fabrication thereof, resulting in high prices of the resultant products.